First of all a nice FOSS bullet hell SHUMP, called Shumpacabra:
Taisei Project is maybe also relevant if you like getting killed ALOT (hehe, insider joke for q).
Previously mentioned Cube Trains (based on Frogatto) has reached version 1.0:
You can support the developer of this nice puzzle game by buying the Expansion-pack for 2 Canadian bucks or more! Another option to try and fund FOSS game development... maybe there will be some sales figures published at some point?
Next on the list: open-source engine reimplementation project Corsix-th:
You will need the original game for the non-free artworks though.
And last but not least: 0 A.D. got a brand new website: www.play0ad.com! Celebrating this they also made some nice video tutorials for total beginners:
Have a look at tutorial 2 and tutorial 3 also.
Signing off :)